How can material selection and design prevent corrosion? Explain which protective coatings can be used for corrosion prevention.

1. Proper selection of material

• Use noble or corrosion-resistant metals (Pt, Au, Ti) or alloys such as stainless steel and cupronickel where the environment is corrosive.
• Use metals of high purity, because impurities form local galvanic cells.
• Choose the material to suit the environment: e.g. Al or stainless steel in moist air, Pb for H₂SO₄, Ni for alkalis, plastics or ceramics where suitable.
• If two different metals must be used together, select metals close together in the galvanic series.

2. Proper designing – design principles

1. Avoid contact of dissimilar metals. If contact is unavoidable, insulate them with a washer or gasket.

      bolt
     ==|==  metal A
     --|--  insulating washer (rubber/plastic)
     ==|==  metal B
      nut
   

2. Keep the anodic area large and the cathodic area small, e.g. copper rivets in a steel plate, not steel rivets in a copper plate.
3. Avoid crevices, sharp corners and recesses where water and dirt collect. Prefer butt welding to riveting or lap joints.

     Bad: riveted lap joint (crevice)      Good: butt weld (smooth)
      ======o======                       ============V============
           ======o======
   

4. Allow complete drainage. Tanks and containers should have sloping or rounded bottoms and drain holes, so no liquid stagnates.

     Bad: flat bottom, liquid left         Good: sloped bottom + drain
     |            |                        |            |
     |~~~~~~~~~~~~|                         \          /
     |____________|                          \___  ___/
                                                 ||  drain
   

5. Avoid sharp bends and abrupt changes in section in pipelines, which cause turbulence, erosion and stress. Use smooth, gradual bends.
6. Keep the structure away from stagnant or uneven aeration, which causes differential-aeration corrosion. Ensure free circulation of air.
7. Design for easy inspection, cleaning, painting and replacement of parts that corrode.
8. Avoid regions of stress concentration, and stress-relieve welded parts by annealing.

3. Protective coatings – the coating isolates the metal from the corrosive environment.

• Metallic coatings
  • Anodic coatings (a more active metal on the base metal), e.g. galvanizing (Zn on Fe). They protect sacrificially even if the coating is broken.
  • Cathodic coatings (a more noble metal), e.g. tinning (Sn on Fe), Ni or Cr plating. They protect only while the coating is unbroken.
  • Methods: hot dipping, electroplating, metal spraying, cladding (e.g. Alclad – pure Al on duralumin), cementation (sherardizing with Zn, chromizing with Cr).
• Organic coatings: paints, varnishes, enamels, lacquers. They form an inert barrier film, and inhibitor pigments such as red lead or zinc chromate add protection.
• Inorganic / chemical conversion coatings: phosphating, chromating, anodising of Al (thick Al₂O₃ film), vitreous enamels.